Rotorcraft drive systems typically include one or more gearboxes. A rotorcraft gearbox—particularly a main rotor gearbox—will often include one or more planetary gear sets. The planetary gear set may include a central sun gear, an outer ring gear, and a plurality of planet gears rotatably coupled to a planetary carrier and configured to “orbit” the sun gear while engaging both the sun gear and the ring gear. Typically, the sun gear receives the torque input to the planetary gear set while the planetary carrier provides the torque output from the planetary gear set. The planetary carrier is often coupled to a rotor mast, such that the torque output from the planetary gear set is applied to the rotor mast.
In some rotorcraft, the planetary carrier and the rotor mast are coupled together using a set of splines. For example, internal splines located on the planetary carrier may transmit the torque output from the planetary gear set to mating external splines located on the rotor mast. Because these splines are typically helical or aligned axially (i.e., substantially parallel to the longitudinal axis of the rotor mast), they provide no support for the planetary carrier in the axial direction.
One solution is to provide carrier-support bearings that support the planetary carrier relative to some fixed structure within the gearbox. The carrier-support bearings are typically rolling-element bearings, such as a ball bearing. However, this solution has several drawbacks. The addition of the carrier-support bearings introduces one or more additional components, which may add complexity to the design, increase weight, and make assembly more difficult. Additionally, the carrier-support bearing will typically necessitate that the planetary carrier include a mating bearing surface, which adds to the cost and complexity of manufacturing the planetary carrier. For instance, the bearing surface will typically require one or more additional machining steps, which might need to be held to tight tolerances. Furthermore, the carrier-support bearing will typically require lubrication, which may require one or more dedicated lube jets to provide oil to the carrier-support bearing. Lubricating the carrier-support bearing may further necessitate additional core passages in the gearbox housing. The addition of lube jets and/or core passages may further negatively impact the gearbox's weight, cost, design complexity, and manufacture and assembly. The carrier-support bearing may require periodic inspection, maintenance, and/or replacement. And, the carrier-support bearing introduces an additional potential failure mode within the gearbox. Other drawbacks associated with such a design will be apparent to one skilled in the art.
Other rotorcraft utilize a rotor mast with an integral planetary carrier. That is, the rotor mast and the planetary carrier are a single, unitary piece having a rotor mast portion and a planetary carrier portion. The planetary carrier is, therefore, supported in the axial direction by the rotor mast portion, which is, in turn, supported in the axial direction by mast bearings. This solution also has several drawbacks. For instance, the gearbox assembly may be more difficult to assemble, ship, and/or store, and it may require more space, because the rotor mast is permanently affixed to the planetary carrier. Manufacture of the rotor mast with integral planetary carrier may be significantly more expensive than the manufacture of two separate parts, particularly because a rotor mast alone might be manufactured from an appropriately sized pipe structure, while a rotor mast with integral planetary carrier might have to be machined from a large billet, casting, or forging, which will potentially result in more machining steps, a longer cycle time, and more material waste. Another disadvantage is that shipping, overhaul, and repair of the mast and/or gearbox becomes more difficult and expensive where the rotor mast and planetary carrier are a single, unitary piece. And, damage or wear to either the rotor mast or the planetary carrier will necessitate overhaul or replacement of the entire unitary piece—which is itself more expensive to repair, overhaul, or manufacture than a separate rotor mast and/or planetary carrier would be.
Consequently, a need exists for a method and apparatus for supporting a planetary carrier in the axial direction, without permanently coupling the planetary carrier to the rotor mast, and without introducing an additional bearing dedicated solely to providing axial support for the planetary carrier. These and other advantages of the present invention will become apparent to one skilled in the art. The embodiments described below, and the inventions set forth in the appended claims, may provide all, some, or none of these advantages.